The Energy Spectrum of Cosmic‐Ray Electrons from 10 to 100 GeV Observed with a Highly Granulated Imaging Calorimeter

Abstract

Cosmic-ray electrons1 have been observed in the energy range from 12 to D100 GeV with a new balloon-borne payload, the Balloon-borne Electron Telescope with Scintillating Fibers (BETS). This is the —rst publication of the absolute energy spectrum of electrons measured with a highly granulated —ber calorimeter. The calorimeter makes it possible to select electrons against the background protons by detailed observation of both the longitudinal and the lateral shower development. The performance of the detector was calibrated by the CERN-SPS accelerator beams: electrons from 5 to 100 GeV, protons from 60 to 250 GeV. The balloon observations were carried out twice, in 1997 and 1998, at the Sanriku Balloon Center (Institute of Space and Astronautical Science) in Japan. The observation time was D13 hr in all at an altitude above 34 km. A total of 1349 electron candidates were collected, and the 628 events with energies above 12.5 GeV, well above the geomagnetic rigidity cutoU of D10 GV, have been used to compose a diUerential absolute energy spectrum at the top of the atmosphere. The energy spectrum is described by a power-law index of 3.00 ^ 0.09, and the absolute diUerential intensity at 10 GeV is 0.199 ^ 0.015 m~2 s~1 sr~1 GeV~1. The overall shape of the energy spectrum in 10 D 100 GeV can be explained by a diUusion model, in which we assume an energy-dependent diUusion coefficient (PE0.3) for an injection spectrum, E~2.4.